1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to the use of data collected from a discharging alkali metal/solid cathode cell to provide a "fuel gauge" for determining the state of charge of the cell. According to the present invention, the use of voltage recovery data from one load to a second, lighter load in a pulse discharging cell, particularly a Li/CF.sub.x cell, is used to estimate the depth-of-discharge (DOD) for the cell. The depth-of-discharge is directly related to the remaining discharge capacity. The use of the pulse discharge data as a fuel gauge according to the present invention is, therefore, particularly useful in an electrochemical cell powering an implantable medical device where the cell may discharge under a light load for extended periods of time interrupted by pulse discharge.
2. Prior Art
In a discharging electrochemical cell, especially an implantable cell, it is desirable to know the amount or quantity of available capacity that remains. This affords the physician an opportunity to schedule surgery for device replacement in a timely and orderly manner without causing undue or unnecessary harm to the patient. Up to now, attempts to determine the charge condition or consumed battery capacity in a pulse dischargeable cell have generally relied on a counter to tabulate the number of pulses delivered by the battery, and hence, the remaining capacity. Representative of these types of devices are U.S. Pat. Nos. 4,556,061 to Barreras et al. and 5,144,218 to Bosscha.
The present invention is not dependent on a raw or cumulative pulse count, but rather, the recovery rate from one load to a second, lighter load or to open circuit voltage (OCV) to determine the depth of discharge for the cell.